Recent advances in semiconductor light sources are changing our daily lives. The classical light sources, such as incandescent and fluorescent light bulbs, are being replaced with light emitting diodes (“LEDs”) that have improved energy efficiency and extended lifetime. Light emitting diode (LED) devices convert ultraviolet light rays or blue light rays emitted from an LED chip into visible light rays that have a longer wavelength. In addition to applications such as indicator lights on home and consumer appliances, audio visual equipment, telecommunication devices and automotive instrument markings, LEDs have found considerable application in indoor and outdoor informational displays. For example, LEDs are used as a backlight unit in liquid crystal displays (“LCDs”) in conjunction with the technical advances in flat panel displays and growing demand in the marketplace for such products.
With the development of efficient LEDs that emit short wavelength (e.g., blue or ultraviolet (UV)) radiation, it has become feasible to produce LEDs that generate white light through down conversion (i.e., phosphor conversion) of a portion of the primary emission of the LED to longer wavelengths. Conversion of primary emissions of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. An unconverted portion of the primary emission combines with the light of longer wavelength to produce white light.
Phosphor conversion of a portion of the primary emission of the LED chip is attained by placing a phosphor layer in an epoxy that is used to fill the reflector cup, which houses the LED chip within the LED lamp. The phosphor is in the form of a powder that is mixed into the epoxy prior to curing the epoxy. The uncured epoxy slurry containing the phosphor powder is then deposited onto the LED chip and is subsequently cured.
The phosphor particles within the cured epoxy generally are randomly oriented and interspersed throughout the epoxy. A portion of the primary radiation emitted by the LED chip passes through the epoxy without impinging on the phosphor particles, and another portion of the primary radiation emitted by the LED chip impinges on the phosphor particles, causing the phosphor particles to emit longer wavelength radiation. The combination of the primary short wavelength radiation and the phosphor-emitted radiation produces white light.
LED technology is inefficient in the visible spectrum. LEDs are becoming increasingly more popular because they are relatively efficient and eco-friendly. For example, LEDs are more widely used as light sources for vehicles, display apparatuses, optical communication systems, and illumination apparatuses. However, because polarization characteristics of light are often used in such application fields, all light emitted from LEDs is not actually used, thereby decreasing optical efficiency. Thus, there is a need in the art for energy efficient LEDs and how to make them.